Inorganic borate compounds have been used as wood preservatives for many years for protection against termites and other wood destroying insects, as well as fungal decay. Soluble borates such as borax, boric acid and disodium octaborate tetrahydrate are well known preservatives in aqueous-based systems for treating solid wood for use in protected environments. However, due to their water solubility they are readily leached from treated wood in exposed environments such as exterior and ground contact applications.
Copper chrome arsenate (CCA) is a leach-resistant wood preservative that has been used for many years to treat solid wood for use in exterior applications. However, due to environmental health and safety issues, and toxicity concerns relating to the constituent metals, particularly arsenic, CCA has come under increasing regulatory pressure and is being phased out of use in many areas. Even compositions containing copper without chromium or arsenic are coming into disfavor for environmental reasons, and thus it is desirable to reduce or eliminate copper content as well.
Solid zinc borate has proven very useful as a preservative for wood composites, where it is added as a solid material during manufacture of the composites. The inherent low solubility of zinc borate makes it resistant to leaching, even in high moisture environments. However, in view of its low solubility, it is not so easy to treat solid lumber with zinc borate. Dev et al. (J. Timb. Dev. Assoc., 1997) describes a two-stage process for treating solid wood with zinc borate which involved impregnating the wood with solutions of borax and zinc in two separate steps. Ammonia-based solutions have been proposed to solubilize metals such as zinc and copper in an attempt to fix borates in wood. U.S. Pat. No. 2,194,827 (Gordon) discloses an aqueous ammonia solution of copper, zinc and borate salts for the treatment of wood.
U.S. Pat. No. 3,974,318 to Lilla discloses a method for fire retarding and preserving wood products, paper, cardboard, boxboard, cloth and other porous materials having a plurality of internal voids, in which a water soluble silicate composition is applied to the porous materials, penetrating into the voids, followed by drying the material. Thereafter, a water soluble metallic salt composition is applied, also penetrating into the voids and reacting in situ to form a water insoluble metallic silicate with a high degree of water of hydration disposed throughout the voids.
Shiozawa (U.S. Pat. No. 5,478,598) discloses a wood preservative composition that includes a first solution having: a copper compound selected from the group consisting of copper borate, copper hydroxide, copper acetate, copper chloride, and copper sulfate; a zinc compound selected from the group consisting of zinc borate, zinc acetate, zinc hydroxide, zinc oxide, zinc chloride, and zinc sulfate; and/or a boron compound selected from the group consisting of boric acid and borax; sodium silicate, and a second solution having rare earth chloride or alkaline earth chloride. The inventor states that the inventive composition can be retained in the wood while the leaching thereof out of the wood is prevented.